PROJECT SUMMARY/ABSTRACT The microtubule associated protein tau has a variety of important cellular functions, most notably stabilizing and organizing microtubules in axons. Tau is also implicated in a host of neurodegenerative disorders such as Pick?s disease, frontotemporal dementia, parkinsonism linked to chromosome 17 and Alzheimer?s disease, which are all characterized by abnormal aggregation of the protein into paired helical filaments and neurofibrillary tangles. Although tau has been the focus of significant recent research, the full range of its normal functions are not understood. Tau is an intrinsically disordered protein and its interactions with microtubules are dynamic, so it has been especially difficult to study the structure of tau-bound to microtubules. Thus, the overall goal of this project is to build a better understanding of tau?s behavior when it is bound to microtubules, including changes in structure and function that precede pathological aggregation. To achieve this end, this project will investigate the properties of tau when it is bound to solid supports in physiologically-relevant conformations, applying an atomic force microscopy (AFM) based assay that interrogates the protein at the nanoscale. Both AFM force spectroscopy and imaging will be used in order to conduct physical-mechanical characterization of normal tau and pro-aggregant mutants, and to examine heterogeneity in assemblies of the protein. This study focuses on three specific aims. The first aim is to characterize normal tau structure and intermolecular interactions when it is surface-bound, studying the six naturally-occurring isoforms of the protein. The second aim will investigate the properties of C-terminal truncated tau mutants, in order to understand the role that the C-terminal plays in normal tau-tau interactions, and to understand how C-terminal truncation promotes abnormal aggregation of tau. In the third aim, tau variants with prevalence in disease will be studied to investigate the origins of their changes in microtubule binding affinity, structure, and propensity for pathological aggregation.